Wnt/??catenin signaling is critically important to development and disease. How this signaling pathway functions normally in vivo remains unclear. At least a dozen different models have been postulated to explain ?-catenin regulation; most are based on studies requiring protein overexpression in highly artificial in vitro assays. Many contradictions may be resolved by a unifying model built on the hypothesis that Wnt signaling in different contexts always uses the same 'core' Wnt pathway, which is fine-tuned by context-specific 'accessory modules'. A new paradigm for Wnt signaling will be developed by determining how components of the core pathway are assembled and regulated by different accessory modules, and what constitutes an accessory module. The goal is to identify these mechanisms by generating a dynamic model of the pathway (Aim 1). The hypothesis is that the core pathway responds to Wnt signal with faster kinetics than accessory mechanisms. An innovative assay has been established in the Drosophila model system to detect interacting Wnt pathway components at molecular resolution and in real time. This has already revealed several highly significant findings, namely the detection, for the first time in cells or in vivo, of the ky regulator in the pathway, termed the 'destruction complex'. This complex is deemed part of the core pathway in contrast to an accessory mechanism represented by the interaction between the receptor Arrow and the destruction complex scaffold Axin, based work by the PI. The molecular mechanism of this accessory module will be probed by functional tests of model-based predictions (Aim 2). In addition, the hypothesis that a set of mutations in Arrow selectively affects Arrow's function in the same accessory module will be tested. Similarities to defects associated with osteoporosis caused by mutations in a human homologue of Arrow, LRP5, raise the possibility that this disorder may be caused by defects in a Wnt accessory mechanism, rather than in the core mechanism, as assumed. Aim 3 tests the hypothesis that the Wnt receptor may initiate signaling by dissociation of the receptor subunits Arrow and Frizzled, contrary to current models for the assembly and activation of this receptor complex. Whether the strong negative regulation of the Frizzled2-Arrow interaction is part of the core pathway or represents a previously unrecognized mechanism for modulating Wnt-dependent processes in a context- specific manner will also be determined. Taken together, the analyses proposed here will significantly advance the understanding of Wnt signaling by generating a dynamic model of normal signaling in vivo at unprecedented resolution. The emerging model is expected to provide a clear distinction between universal regulation by the core pathway and tissue-specific modulation of the Wnt signal by accessory mechanisms. Such a paradigm shift will reveal, in many instances, that Wnt-associated diseases are defective in tissue-specific accessory mechanisms and such a distinction will greatly facilitate the development of targeted therapies.